AIMS Provenance: ATLAS

Draft for review. This page describes the current AIMS handling of ATLAS difference-image detections.

Source summary

The Asteroid Terrestrial-impact Last Alert System (ATLAS) is a global all-sky wide-field survey operated by the University of Hawaiʻi to search continuously for hazardous near-Earth objects. The ATLAS PDS bundle overview describes ATLAS images and source catalogs as archival products useful beyond solar-system science.

ATLAS difference products include:

• reduced images;
• difference images (.diff.fz);
• reduced-image detection catalogs;
• difference-image detection catalogs (.ddc).

The ATLAS PDS documentation describes .ddc catalogs as detections found in the difference image by the ATLAS pipeline. The tphot source finder supports fixed, free, trailed, and streaked PSF fits, which is important for asteroids and comets that can be trailed in an exposure. The vartest classifier assigns probabilities for known asteroids, unknown asteroids/transients, variables, and artifact classes on a 0—999 scale.

How AIMS accesses ATLAS

AIMS operational ingestion uses ATLAS-provided difference products. For each daily partition, AIMS retrieves exposure headers and .ddc difference-detection catalogs for the ATLAS sites that have data on that ATLAS night.

AIMS uses the FITS header values for exposure timing and filter metadata, and the .ddc rows for detection position, photometry, PSF-shape information, and classifier probabilities.

AIMS date range and observatories

• AIMS partition start: 2015-06-18.
• AIMS treats ATLAS as active/continuing.

AIMS maps ATLAS site/camera prefixes to MPC observatory codes:

AIMS filtering

AIMS filters each .ddc catalog to moving-object-like rows before normalization. A row is kept only when all of the following are true:

• Ptr > 200 or Pkn > 200;
• Dup >= 0;
• dmag >= 0.

Here Ptr is the ATLAS probability that the detection is an unknown asteroid/transient, and Pkn is the probability that it is a known asteroid. The ATLAS PDS overview describes the probability convention as 0 for zero probability and 999 for unity.

Dup is the ATLAS duplicate-selection class. Negative duplicate values indicate rows rejected in favor of a better nearby measurement; AIMS excludes those rejected duplicates.

AIMS also performs deterministic duplicate collapse when multiple kept rows produce the same AIMS observation id. The winner is chosen by:

1. higher detection-class rank, preferring streaked then trailed then free then fixed detections;
2. greater elongation;
3. lower dmag;
4. higher moving-object confidence from max(Ptr, Pkn);
5. stable first-seen row order.

AIMS-derived and normalized values

ATLAS astrometric uncertainty approximation

AIMS derives a per-axis astrometric uncertainty because the .ddc rows do not provide explicit RA/Dec sigmas in the normalized source path. The current approximation is:

fwhm_proxy = sqrt(major^2 + minor^2)
snr = 1.0857 / dmag
sigma = (fwhm_proxy / snr) / 3600 degrees

Rows with dmag == 0 are assigned dmag = 0.001 for this calculation to avoid division by zero and to provide a conservative nonzero uncertainty.

Known limitations and interpretation notes

• AIMS ingests a filtered subset of ATLAS difference detections, not all ATLAS detections or images.
• The Ptr/Pkn threshold is an AIMS selection threshold; it is not a final object classification.
• The astrometric uncertainty is an AIMS approximation derived from ATLAS PSF-shape and photometric-uncertainty fields.
• ATLAS duplicate behavior is source-specific; AIMS intentionally follows the ATLAS duplicate-selection signal and then applies an additional deterministic tie-break if needed.
• Public PDS documentation is useful provenance for ATLAS products, but AIMS operational access may use ATLAS-provided internal delivery paths rather than the public PDS archive.

Source links

• ATLAS survey home
• ATLAS PDS bundle overview PDF
• NASA Planetary Data System Small Bodies Node